The present invention relates to phenyl-endcapped depolymerizable polymers having increased depolymerization threshold temperatures and reduced residue after depolymerization. More particularly, the invention relates to phenyl-endcapped poly(methyl methacrylate) and poly(alpha-methyl styrene) and their use as lift-off materials and ceramic binders.
Poly(methyl methacrylate) and poly(alpha-methyl styrene) are thermally depolymerizable polymers. These polymers depolymerize by "unzipping," that is they essentially undergo a complete reverse of polymerization, regenerating the gaseous monomer or monomers from which the unzippable polymer was formed.
For use as lift-off materials and binders, an optimal thermally depolymerizable polymer should have the following properties:
1) It should not depolymerize below a given threshold temperature that is above the highest temperature that the structure is exposed to during intermediate process steps. PA0 2) It should not leave any residue after depolymerization. PA0 3) It should have good sheet or film-forming properties.
Lift-off processes are well known in the art, especially for applying metallization patterns to semiconductors. In such a process, a sacrificial layer is deposited and patterned with the inverse of the desired metallurgy pattern. Following blanket metallization, the sacrificial layer is dissolved, "lifting off" the metal in the undesired areas. U.S. Pat. Nos. 4,519,872, 4,539,222 and 4,456,675 describe lift-off layers comprising poly(methyl methacrylate) or poly(alpha-methyl styrene). These films undergo rapid weight loss at temperatures of 275.degree.-315.degree. C. This creates a problem when argon sputter cleaning at 240.degree. C. is used prior to metallization, because the polymers begin to unzip at this temperature.
Binders are normally comprised of simple solvent soluble thermoplastic organic polymers having good film-forming properties which are nonvolatile at moderate temperatures but which will volatilize with other constituents of the resin system on firing of the green sheet to the final sintered state. A commonly used binder resin is poly(vinyl butyral). However, where low temperature systems, such as glass ceramic substrates, are used, the maximum temperature for binder removal is much lower due to the coalescence of the glass particulate at about 800.degree.-875.degree. C. Thus, after the glass has coalesced, any remaining binder residue will become entrapped in the glass body. Failure to completely remove the binder in the initial stages of the heat treatment process can result in the evolution of gas during the final sintering stage. This gives rise to pores in the material or, in extreme cases, to blow holes.
Glass ceramic substrates are typically used with copper metallurgy. Copper metallurgy precludes the use of oxidizing ambients during binder burn-off. As disclosed in U.S. Pat. No. 4,234,367, it has been found that conventional binder resins, such as poly(vinyl butyral), are not easily burnt out in these non-oxidizing atmospheres. This can result in black or darkened substrates that are not fully sintered. The black or darkened color is generally attributed to carbon residue. The carbon remaining in the ceramic can form conduction paths which lead to lower insulation resistance by many orders of magnitude and to increased dielectric losses.
The use of unzipping polymers such as free radical polymerized poly(methyl methacrylate), poly(alpha-methyl styrene) and polyisobutylene, has been disclosed in IBM Technical Disclosure Bulletin, July 1979, p. 542 to Anderson, et al. and in U.S. Pat. No. 4,598,107. These polymers have cleaner burn-out and minimal residue formation in an inert atmosphere as compared to poly(vinyl butyral). A problem with these polymers is that as a result of the free radical polymerization process by which they are formed, there is rather limited control in molecular weight distribution and end groups. Furthermore, the reactive terminal vinyl group promotes cross-linking which leads to carbonaceous residues. This residue will not be removed in low temperature or non-oxidizing systems.
U.S. Pat. No. 4,550,061 discloses the use of alpha-substituted styrene polymers or polymers derived from alpha-substituted acrylate monomer as binders for electro-erosion printing media. The materials have decreased residue after decomposition (less than 2%).
U.S. Pat. No. 4,474,731 discloses a process for the removal of carbon residues formed during sintering of ceramics.